Isomerization catalyst and use thereof in alkane/cycloalkane isomerization

ABSTRACT

A catalyst composition is prepared by a method comprising impregnating alumina with at least one platinum compound, followed by calcining, reducing treatment, and heating with gaseous aluminum chloride and gaseous titanium tetrachloride. The thus-prepared catalyst composition is employed in the isomerization of saturated C 4  -C 8  hydrocarbons (alkanes and/or cycloalkanes), preferably n-butane.

This application is a division of application Ser. No. 08/340,497; filedNov. 15, 1994; now U.S. Pat. No. 5,543,374.

BACKGROUND OF THE INVENTION

In one aspect, this invention relates to the preparation of aplatinum-containing catalyst composition. In another aspect, thisinvention relates to the use of this composition as a catalyst forisomerizing saturated C₄ -C₈ hydrocarbons.

Supported platinum/chlorine-containing catalyst compositions and theiruse in alkane isomerization reactions are well known, and are describedin the patent literature, e.g., in U.S. Pat. Nos. 5,004,859 and4,149,993. However, there are ever present incentives for thedevelopment of new, more active Pt/Cl-containing catalyst compositionsand new methods of preparing them.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a novel method forpreparing a supported, Pt/Cl-containing catalyst composition. It isanother object of this invention to provide a novel catalyst compositionprepared by this preparation method. It is a further object of thisinvention to employ this novel catalyst composition in reactions forisomerizing saturated C₄ -C₈ hydrocarbon. Other objects and advantageswill become apparent from the detailed description and the appendedclaims.

In accordance with this invention, a method of preparing a solidplatinum- and chlorine-containing composition comprises:

(a) impregnating alumina with at least one platinum compound,

(b) heating the impregnated alumina obtained in step (a) at atemperature of about 300°-650° C. for a time period of at least about 5minutes;

(c) heating the calcined material obtained in step (b) with a reducinggas; at a temperature of at least about 100° C. for a time period of atleast about 5 minutes; and

(d) treating (in any order) the material obtained in step (c) withgaseous aluminum chloride and gaseous titanium tetrachloride at atemperature of about 200°-800° C. for a time period of at least about 10minutes, wherein the molar ratio of AlCl₃ to TiCl₄ is in the range ofabout 5:1 to about 15:1.

Also in accordance with this invention, a catalyst composition isprovided which has been prepared by one of the preparation methodsdescribed above.

Further in accordance with this invention, at least one saturated feedhydrocarbon containing 4-8 carbon atoms per molecule selected from thegroup consisting of alkanes and cycloalkanes is isomerized to at leastone corresponding saturated hydrocarbon isomer in the presence ofhydrogen gas and a catalyst composition which has been prepared by thepreparation method described above.

DETAILED DESCRIPTION OF THE INVENTION

(A) Catalyst Preparation

Any suitable alumina material can be used in steps (a) of thepreparation method of this invention. Suitable aluminas include (but arenot limited to) hydrated aluminas (such as boehmite, pseudoboehmite,bayerite), alpha-alumina, beta-alumina, gamma-alumina, delta-alumina,eta-alumina and theta-alumina, preferably gamma-alumina. The aluminamaterial generally has a surface area (determined by the BET method ofBrunauer, Emmett and Teller employing N₂) of about 100-400 m² /g, a porevolume (measured by nitrogen intrusion porosimetry) of about 0.2-1.0 cm³/g, and a particle size of about 8-200 mesh. The alumina particles canbe spherical, cylindrical, trilobal, or can have any other suitableshape. The presently preferred alumina particles are cylindricalextrudates. Washing of the alumina material with an acidic solution(such as an aqueous NH₄ Cl solution) should be avoided. Chemically boundsulfur (e.g., as sulfate) is to be substantially absent from the alumina(i.e., bound S should not be present or be present at a level of lessthan about 0.1 weight-% S).

Any suitable platinum compound which is water-soluble can be used instep (a) oft he preparation method of this invention. These compoundsare well known and include (but are not limited to) platinum dichloride,platinum tetrachloride, hexachloroplatinic(IV) acid, ammoniumhexachloroplatinate(IV), tetrammineplatinum(II) chloride,tetrammineplatinum(II) carbonate, tetrammineplatinum(II) hydroxide,dichlorodiammineplatinum(II), tetrachlordiammineplatinum(IV),platinum(II) nitrate, platinum(IV) nitrate, hexammineplatinum(II)nitrate, hexammineplatinum(IV) nitrate, diammineplatinum(IV) nitrite,diammineplatinum(II) oxalate, other complexes or coordination compoundsof divalent and tetravalent platinum, and mixtures of two or more thantwo of these Pt compounds. Presently preferred is hexachloroplatinicacid, H₂ PtCl₆.

The alumina material can be impregnated with at least one dissolvedplatinum compound in any suitable manner. Preferably, an aqueoussolution of Pt compound(s) is used. Generally, the concentration of theplatinum compound(s) in the aqueous impregnating solution is about 1-2mole/l. The weight ratios of dissolved platinum compound(s) is such asto incorporate about 0.1-0.6 (preferably 0.2-0.4) weight-% Pt into thealumina material.

In step (b) of the preparation method of this invention, thePt-impregnated alumina material obtained in step (a) is calcined at atemperature of about 300°-650° C. (preferably 450°-600° C.) for a timeperiod of about 0.5-20 hours (preferably about 2-4 hours). Thiscalcining step can be done in an inert atmosphere (i.e.,N₂, He, Ar) orin an O₂ -containing atmosphere (e.g., air). Preferably, thePt-impregnated alumina material is dried (e.g., at about 80°-150° C.)before step (b) carried out.

In step (c) of the preparation method of this invention, the calcinedPt-containing alumina is contacted with at least one gaseous reducingagent. Examples of such reducing gases include (but are not limited to)hydrogen and carbon monoxide. Presently preferred is H₂. Generally, thetemperature in step (c) is about 200°-800° C. (preferably about300°-600° C.) and the contacting time is about 0.2-10 hours (preferablyabout 0.5-5 hours).

In step (d), the reduced material obtained in step (c) is heated withgaseous AlCl₃ and gaseous TiCl₄. Preferably, hydrogen gas is alsopresent during heating step (d). It is within the scope of thisinvention to carry out the treatment with AlCl₃ and TiCl₄ in any orderor essentially simultaneously, the latter being preferred. If thechloriding treatment with TiCl₄ and AlCl₃ is carded out sequentially, itis preferred to have H₂ gas present in each chloriding step. Step (d) iscarded out at a temperature of about 200°-800° C. (preferably about400°-700° C., more preferably about 550°-650° C.) for a period of timeof about 0.2-20 hours (preferably about 0.5-2 hours), at a hydrogenpressure of about 100-1000 psig (preferably 300-700 psig). The molarratio of AlCl₃ to TiCl₄ generally is in the range of about 5:1 to about15:1, preferably in the range of about 7:1 to about 13:1, morepreferably in the range of about 8:1 to about 12:1, and most preferablyin the range of about 9:1 to about 11:1.

The finished catalyst composition generally contains about 0.1-0.3(preferably about 0.15-0.2) weight-% Ti, about 0.1-1 (preferably about0.2-0.4) weight % Pt and about 2-6 (preferably about 2.5-4) weight % Cl.The surface area, pore volume, shape and particle size of the finishedcatalyst composition are approximately the same as those of the aluminastarting material (recited above).

(B) Isomerization Process

The catalyst of this invention is generally employed in theisomerization of saturated C₄ -C₈ hydrocarbons (preferably normalalkanes). Examples of suitable feed hydrocarbons include (but are notlimited to) normal butane, normal pentane, normal hexane, normalheptane, normal octane, cyclohexane, methylcyclopentane, cycloheptaneand methylcycloheptane (more preferably n-butane), generally in thepresence of hydrogen. These so-called hydroisomerization processes arewell known and have been described in the patent literature (e.g., inU.S. Pat. Nos. 4,149,993 and 5,004,859). Generally, hydrogen is mixedwith the saturated hydrocarbon feed to form a feed mixture which iscontacted with the isomerization catalyst of this invention contained inan isomerization zone. The concentration of the hydrogen in the feedmixture during this contacting step shall be such as to provide ahydrogen:hydrocarbon molar ratio of at least about 0.01:1, generallyabout 0.01:1 to about 5:1, preferably about 0.02:1 to about 2:1. Thebasic isomerization reaction conditions are well known and can be variedto achieve maximum conversion of the feed hydrocarbon to the desiredisomer(s) in any manner known in the art. Also, the recovery of theproduct isomer(s) from the reaction mixture can be carded out by anysuitable separation technique, such as fractional distillation.Isomerization of normal butane (n-butane) to isobutane is the presentlypreferred reaction carried out with the catalyst composition of thisinvention.

Generally, the saturated feed hydrocarbon and H₂ are contacted with thecatalyst (preferably present in a fixed bed) at a reaction temperatureof at least about 200° F., preferably at a temperature of about200°-500° F. In the case of n-butane isomerization, the preferredtemperature is about 250°-400° F. Generally, the liquid hourly spacevelocity of the saturated hydrocarbon feed stream, i.e., cc of liquidfeed hydrocarbon per cc of catalyst per hour, is about 0.1 to about 15.Generally, the reaction pressure is within the range of 200 psig toabout 1500 psig in the isomerization zone. The gas hourly space velocityof the hydrogen feed stream is generally about 10-2,000 (preferablyabout 50-1000) cc H₂ per cc catalyst per hour (so as to give theabove-recited H₂ :hydrocarbon ratio). In order to activate the catalystand to retard its deactivation during the isomerization reaction, about0.001 to about 1 weight percent chloride is frequently added to thealkane feed, generally in the form of at least one chloroalkane,preferably carbon tetrachloride, chloroform, ethyl chloride or isopropylchloride.

When the catalyst employed in the hydroisomerization process has lostits activity to the extent that the desired alkane conversion can nolonger be attained at the desired reaction temperature, the catalyst canbe reactivated by mining off the flow of the saturated feed hydrocarbonwhile maintaining the flow of the H₂ stream through the isomerizationcatalyst, generally at about the same temperature and the same gashourly space velocity of H₂ as in the isomerization reaction. In thepreferred reactivation mode, a reducing gas stream comprising hydrogenand, optionally, a chloriding agent is passed through the partiallydeactivated isomerization catalyst bed at a temperature of about50°-400° F. (preferably about 250°-330° F.) and a GHSV (gas hourly spacevelocity) of about 10-2,000 cc H₂ per cc catalyst per hour (morepreferably about 50-950 cc/cc/hour), for a time period of about 2 hoursto about 10 days (more preferably about 5 hours to about 7 days). Theactivated catalyst is then redeployed in the hydroisomerization processof this invention.

The following examples are presented to further illustrate the presentinvention and are not to be construed as unduly limiting the scope ofthis invention.

EXAMPLE I

This example illustrates the preparation of various alumina-supportedplatinum catalysts. This example illustrates the preparation of variouschlorided platinum-impregnated alumina materials.

Catalyst A was prepared as follows: about 15 grams of alumina (providedby Criterion Catalyst Company, Houston, Tex.; sulfur content: 0%) wasimpregnated (by incipient wetness) with an aqueous solution ofhexachloroplatinic acid (containing 1.0 gram of H₂ PtCl₆, 65.6 grams ofwater and 1.4 grams of HCl). The thus-impregnated alumina wassubstantially dried by means of an aspirator pump for 1-2 hours, heatedin a helium gas stream at 525° C. for about 1 hour, heated an oxygen gasstream at 525° C. for several hours, cooled in a helium gas stream to425° C., heated in a hydrogen gas stream at 425° C. for about 2 hours,and then cooled to 150° C.

Thereafter, a downflow stainless steel reactor was charged with 14.8grams (20 cc) of the Pt-impregnated alumina and a top layer of Alundum®(inert alumina beads having a surface area of less than 1 m² /g). Thereactor and its contents were heated (at a rate of 10° C. per minute) toabout 595° C. while helium gas flowed through the reactor and itscontents at a rate of 1.0 liter/minute. The helium gas flow wasmaintained at a temperature of about 595° C. for about 10 minutes. Thenhydrogen gas was passed through the reactor and its contents at a rateof 0.3 liter/minute and a reactor temperature of 595° C., for a timeperiod of 5-10 minutes. Thereafter a powder mixture of 1.75 grams. ofAlCl₃ to TiCl₄ (at a molar AlCl₃ :TiCl₄ ratio of about 10:1) wasintroduced with the dry hydrogen gas stream and was deposited on theAlundum® layer on top of the Pt/Al₂ O₃ material in the reactor. The H₂gas stream passed through the AlCl₃ /TiCl₄ /Alundum® layer (maintainedat a temperature of 180°-280° C.) and then through the reactorcontaining Pt/Al₂ O₃ material (maintained at a temperature of about 595°C.) for a time period of 1 hour during which AlCl₃ and TiCl₄ sublimedonto and interacted with the Pt/Al₂ O₃ material. Then the H₂ gas streamwas replaced with a helium/hydrogen gas stream containing 70 volume-% Heand 30 volume-% H₂, and the reactor was cooled to 150° C. The finishedcatalyst contained 0.32 weight-% Pt and 3-4 weight-% Cl.

Catalyst B was essentially a duplicate of Catalyst A. Catalyst Bcontained 0.32 weight-% Pt, and 3.6 weight-% Cl.

Catalyst C was prepared essentially in accordance with the procedure forCatalyst A, except that TiCl₄ was introduced about 10 minutes prior tothe introduction of AlCl₃. The AlCl₃ :TiCl₄ molar ratio was 10:1.Catalyst C contained 0.32 weight-% Pt, and 2.7 weight-% Cl.

Catalyst D was prepared in accordance with the procedure for Catalyst A,except that the molar ratio of AlCl₃ to TiCl₄ in the mixture of the twocompounds (which was placed on top of the Alundum® layer) was 20:1.Catalyst D contained 0.32 weight-% Pt, and 3.1 weight-% Cl.

Catalyst E was essentially a duplicate of Catalyst D (AlCl₃ :TiCl₄ molarratio: 20:1). It contained 0.32 weight-% Pt and 3.0 weight-% Cl.

Catalyst F was prepared essentially in accordance with the procedure forCatalyst A, except that only AlCl₃ (in lieu ofAlCl₃ /TiCl₄) was employedas the chloriding agent. Catalyst F contained 0.32 weight-% Pt and 3.5weight-% Cl.

Catalyst G was prepared essentially in accordance with the procedure forCatalyst A, except that only TiCl₄ (in lieu of AlCl₃ TiCl₄) was employedas the chloriding agent. Catalyst G contained 0.32 weight-% Pt and 3.7weight-% Cl.

EXAMPLE II

This example illustrates the use of the catalyst materials described inExample I in the isomerization of n-butane.

20 cc of each catalyst was placed in a stainless steel reactor tubehaving an inner diameter of 1 inch and a length of 28 inches. The steelreactor tube was heated to 138° C. A stream of hydrogen gas was passedthrough the catalyst bed at a rate of 1.34 cubic feet per hour. Thereactor pressure was about 500 psig, Liquid n-butane was introduced at arate of 78 about cc/hour (liquid hourly space velocity:3.9 cc/cccatalyst/hour), while the flow of the hydrogen gas stream was maintainedat 1.35 ft³ /hour so as to provide a molar ratio of H₂ to n-butane ofabout 50:1. After the hydrogen/n-butane mixture had passed through thecatalyst bed at the above conditions for about 10 minutes, carbontetrachloride was injected into this feed mixture at a rate of 16microliters per hour for a time period of up to about 24 hours.Thereafter, the CCl₄ feed rate was reduced to 6 microliters per hour,and the test was continued. The isomerization product was analyzed bymeans of a gas chromatograph. Pertinent catalyst preparation parametersand isomerization test results (obtained at comparable reactionconditions) are summarized in Table I.

                                      TABLE I                                     __________________________________________________________________________                                      % Isobutane                                         Treatment of Pt/Al.sub.2 O.sub.3                                                          AlCl.sub.3 :TiCl.sub.4                                                               Wt - % Cl in                                                                         in C.sub.4 Isom                             Run                                                                              Catalyst                                                                           with AlCl.sub.3                                                                     with TiCl.sub.4                                                                     Mol Ratio                                                                            Catalyst                                                                             Product.sup.1                               __________________________________________________________________________    1  A    Yes   Yes   10:1   N/A    20.0                                        2  B    Yes   Yes   10:1   3.6    8.6.sup.2                                   3  C    Yes   Yes   10:1   2.7    19.3                                        4  D    Yes   Yes   20:1   3.1    15.3                                        5  E    Yes   Yes   20:1   3.0    12                                          6  F    Yes   No    --     3.5    16.5                                        7  G    No    Yes   --     3.7    8.5                                         __________________________________________________________________________     .sup.1 based on isomerization product excluding H.sub.2, produced after       about 20 hours; isomerization conditions of all runs: amount of catalyst:     about 20 cc (about 15 g); reaction temperature: about 138° C.;         reaction pressure: about 500 psig; liquid nbutane feed rate: about 80         cc/hour; H.sub.2 feed rate: about 1.3 ft.sup.3 /hr; H.sub.2 :nbutane mol      ratio: about 10:1                                                             .sup.2 result is considered erroneous; it is believed that a hydrogen gas     dryer did not work properly and that moisture was introduced into the         system.                                                                  

Test data in Table I show that generally a more active isomerizationcatalyst was obtained when both aluminum chloride and titaniumtetrachloride were used as chloriding agents, at a molar AlCl₃ :TiCl₄ratio of about 10:1.

Additional tests (not described in detail herein) indicated that washingof the Criterion alumina with a 1 molar aqueous NH₄ Cl solution forabout 1-6 hours prior to the catalyst preparation consistently producedcatalysts of low isomerization activity. Thus, this pretreatment ofalumina should be avoided. Another test (not described in detail herein)showed that the presence of about 0.5 weight-% of chemically boundsulfur in the alumina caused the resulting chlorided Pt/Al₂ O₃ catalystto have low isomerization activity. Thus, chemically bound sulfur shouldbe substantially absent from the alumina starting material.

Reasonable variations, modifications, and adaptations for various usagesand conditions can be made within the scope of the disclosure and theappended claims without departing from the scope of this invention.

That which is claimed:
 1. A process for isomerizing saturatedhydrocarbons which comprises contacting at least one saturated feedhydrocarbon containing 4-8 carbon atoms per molecule selected from thegroup consisting of alkanes and cycloalkanes at effective isomerizationconditions with hydrogen gas and an effective isomerization catalyst soas to produce at least one product isomer, wherein said effectiveisomerization catalyst has been prepared by a method comprising thesteps of(a) impregnating alumina with at least one platinum compound;(b) heating the impregnated alumina obtained in step (a) at atemperature of about 300°-650° C. for a time period of at least 5minutes; (c) heating the calcined material obtained in step (b) with areducing gas at a temperature of at least about 100° C. for a timeperiod of at least about 5 minutes; and (d) treating the materialobtained in step (c) with gaseous aluminum chloride and gaseous titaniumtetrachloride at a temperature of about 400°-700° C. for a time periodof at least about 10 minutes, wherein the molar ratio of AlCl₃ to TiCl₄is in the range of about 5:1 to about 15:1.
 2. A process in accordancewith claim 1, wherein hydrogen gas is also present in the treating step(d).
 3. A process in accordance with claim 2, wherein chemically boundsulfur is substantially absent from said alumina.
 4. A process inaccordance with claim 3, wherein said alumina has not been contactedwith an acidic solution prior to step (a).
 5. A process in accordancewith claim 2, wherein the conditions of step (a) are such as toincorporate about 0.1-1 weight-% Pt into said alumina.
 6. A process inaccordance with claim 2, wherein heating step (b) is carried out at atemperature of about 450°-600° C. for about 0.2-20 hours.
 7. A processin accordance with claim 2, wherein step (c) is carried out withhydrogen gas at a temperature of about 300°-600° C. for a time period ofabout 0.2-10 hours.
 8. A process in accordance with claim 2, whereinstep (d) is carried out for a time period of about 0.2-20 hours, at atemperature of about 400°-700° C. and a hydrogen pressure of about100-1,000 psig.
 9. A process in accordance with claim 8, wherein step(d) is carried out at a molar ratio of AlCl₃ to TiCl₄ of about 7:1 toabout 13:1.
 10. A process in accordance with claim 9, wherein step (d)is carried out for a time period of about 0.5-2 hours, at a temperatureof about 550°-650° C., a hydrogen pressure of about 300-700 psig and amolar ratio of AlCl₃ to TiCl₄ of about 8:1 to about 12:1.
 11. A processin accordance with claim 1, wherein said effective isomerizationcatalyst comprises about 0.1-1 weight-% Pt, about 0.1-0.3 weight-% Tiand about 2-6 weight-% Cl.
 12. A process in accordance with claim 11,wherein said effective: isomerization catalyst comprises about 0.2-0.4weight-% Pt, about 0.15-0.2 weight-% Ti and about 2.5-4 weight-% Cl. 13.A process in accordance with claim 11, wherein said at least onesaturated feed hydrocarbon is n-butane.
 14. A process in accordance withclaim 13, wherein said effective isomerization conditions comprise amolar ratio of hydrogen to n-butane is about 0.1:1 to about 5:1, areaction temperature of about 200°-500° C. and a reaction pressure ofabout 200-1500 psig.
 15. A process in accordance with claim 2, whereinsaid effective. isomerization catalyst comprises about 0.1-1 weight-%Pt, about 0.1-0.3 weight-% Ti and about 2-6 weight-% Cl.
 16. A processin accordance with claim 15, wherein said effective isomerizationcatalyst comprises about 0.2-0.4 weight-% Pt, about 0.15-0.2 weight-% Tiand about 2.5-4 weight-% Cl.
 17. A process in accordance with claim 15,wherein said at least one saturated feed hydrocarbon is n-butane.
 18. Aprocess in accordance with claim 17, wherein said effectiveisomerization conditions comprise a molar ratio of hydrogen to n-butaneis about 0.1:1 to about 5:1, a reaction temperature of about 200°-500°C. and a reaction pressure of about 200-1500 psig.